Related Questions:
How does 50% extra xp weekend stack with other bonuses?
Extra experience from equipment
While there is some good information in the answers to the above questions, I feel none of them give the complete picture.
In Diablo 3, there are a number of different ways to increase your exp gain. Assuming your level is fixed (at 70, most likely), we have the following effects:
- "Strength in Numbers" buff (group bonus)
- "Increases bonus experience by ..%" from equipment and/or ruby in head slot
- "Monster kills grant +… experience" from equipment and/or Gem of Ease
- Difficulty bonus (a % bonus)
These all modify the base experience received from slaying a monster (again, assuming for simplicity that player level isn't taken into account). For simplicity, I will refer to them as (in the same order).
- Group%
- Bonus%
- Bonus
- Difficulty%
Now, how these work on their own is pretty straightforward, but how do they stack when two or more are present? Knowing the exact way they stack could make a big difference when evaluating different item properties.
For example, having "Monster kills grant +… experience" on a few pieces of gear could make a big difference if the calculation is something like:
Net gain = (Base + Bonus) * Group% * Bonus% * Difficulty%
However, it would be almost useless at higher difficulties if the formula was more like:
Net gain = Base * Group% * Bonus% * Difficulty% + Bonus
The names aren't very descriptive, but if I had to guess, my guess would be something like:
Net gain = (Base + Bonus) * (Bonus% + Difficulty%) * Group%
However, this is mostly guesswork. Is there any official source (or someone doing a bunch of science) that clears this up? I also remember someone saying there was a difference in how experience gain was calculated in Greater Rifts compared to the rest of the game. Any truth to this?
(Note: When multiplying by X%, I naturally mean multiplication by 1.00 + 0.01*X. Multiplying by (X% + Y%) refers to a multiplication by 1.00 + 0.01*(X + Y))
Edit: While broad in scope, the question primarily applies to the current (as of August 2015) state of the game. So patch 2.2.1 primarily (which also applies to season 4/patch 2.3.0, since there are no changes to the exp formula planned, only base exp balancing and bonus exp being averaged over the party).
Best Answer
Data from this post suggests that Bonus% is a separate multiplier in Greater Rifts, rather than additive with game difficulty. I've seen multiple people say that Bonus% is additive with game difficulty outside of Greater rifts, but I'll restrict the rest of this post to GR.
As you mentioned, Bonus% is (sum of all experience gear across the party)/(number of party members), since experience gear is divided evenly across the party in 2.3.
With perfect rolls on Leoric's Crown and Ring (note this gets 45% at level 70, not 35% as the post mentions), you're looking at (82+30+45+20+50)/4 = 56.75% bonus exp per exp-geared char in a 4-man party.
In his runs, adding one exp person brought exp up 63.95% from ~8.6M to ~14.1M. Adding two exp people brought exp up 73.84% (per exp-geared person, 147.65% total) from ~8.6M to ~21.3M. He only ran 10 runs in each test set, and rifts vary widely, though it could be he forgot to mention other changes in the gear, such as additional rubies in helms of non-exp chars or using Gem of Ease on the measured character.
Patch 2.1.2 brought the change:
All of this leads me to believe that this is the correct formula for Greater Rifts:
I've seen reports that it's different for outside of Greater Rifts, which changed in Season 2 or 3, but it wasn't in patch notes.
As for how useful the +Bonus is, I think they've purposely scaled Base such that it is extreme at low levels and barely noticeable at high levels. If the percentage discrepancies above are due to Gem of Ease, you're talking about a 5-16% exp difference, which is worth it.
Also I believe experience shrine is a multiplier as well, but my sources for this are outdated, so it could have changed.